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Tess Nichol talks to Laura Mucha about the truth when it comes to sex and cheating.

For a book about love, the opening pages of Love, Factually are a bit of a downer.

A reader curious to learn about the life affirming secrets of love will instead discover mere moments after cracking the spine that any coupled person they meet is more likely than not to have done the dirty on their partner.

Studies into infidelity have varied results but those at the higher end found nearly three-quarters of men and a similar number of women admitted to cheating ona spouse.

Studies at the lower end found just 14 per cent of men and 10 per cent of women confessed to cheating - but those numbers often rose again in therapy sessions where, after gentle probing, partners admitted infidelity.

The huge range in results is likely due to the fact people are unwilling to admit to cheating because of how culturally unacceptable it is. But before optimistically deciding the bleaker studies were outliers, you should know one of the most reliably representative US studies found 66 per cent of 7236 married men admitted to sleeping with someone who wasn't their spouse.

What the hell? Is love dead? Was it ever real to begin with? And why start with such grim numbers?

Laura Mucha, who interviewed hundreds of strangers for her book, thinks first and foremost we need to get straight the difference between love and lust - two feelings people are apt to muddle.

She says infidelity is a good starting point for discussing this. In fact, before she started the project Mucha says she didn't really understand the distinction either.

"I think I often thought when I was in lust I was in love and then when it faded I thought 'Ugh, no.' And it happened enough times that you would thought I would have learned, but apparently not."

The thrill of lust - the sexual intensity of a new lover, how intoxicated you feel by their smell and touch, is a drive we need to get us to reproduce, but it doesn't necessarily stick around in a long term relationship. It might fade, or come and go over the years.

What is hopefully revealed when lust fades is romantic and companionate love, which involves intimacy, honesty, vulnerability and commitment. This kind of love is about fulfilling the desire most people have which is to partner with someone who is above all else kind - a nearly universal and rather comforting finding.

"All across the world people said kindness and understanding - more than intelligence or excitingness or whatever," Mucha said.

Over 10 years Mucha interviewed somewhere between 300 and 400 people about their thoughts on love, the project spanning more than 30 countries and every continent, including Antarctica.

That meant getting about three strangers a month to speak freely on a deeply intimate topic, every month for a decade. It's an astonishing number, but talking to Mucha you get the feeling she may have struck up these conversations whether she was writing a book or not.

Bubbly, lively, endlessly curious and exceedingly thoughtful, Mucha possesses that trait all journalists would kill for: the knack for getting a total stranger to open up almost immediately and spill their guts.

In the course of our two-hour conversation I end up divulging a lot of my romantic hang ups - not because I have to, that's well outside the remit of a normal interview, they just sort of slip out as we chat. I tell her how shocked I was by the cheating statistics. "The levels of infidelity were higher than I expected and that feels ... depressing," I say.

Mucha laughs for a long time, then stops herself. "Sorry," she says, cheekily - she's obviously dealt with this reaction before. She's concluded that the fact we're all so shocked by something which is clearly quite common means as a society we are not great at dealing with the reality of human nature. Namely, that monogamy is hard, and we're apt to judge people harshly for crossing strict societal boundaries.

"We don't talk about infidelity because everyone is so judgmental about it, there's no honest conversation about it," Mucha says.

"I think it comes down to a paradox, a really fundamental paradox which is unsolvable. Our long term relationships are really important - I mean like friendships, family, romantic relationships - are key to providing us a sense of safety in the world. At the same time, we have this reproductive drive, which has got to be really powerful because we'd die out as a species without it, so the kind of challenge is this need to safety and security but also this need for the thrill of everything that happens when you're lusting after someone."

Laura Mucha, the author of Love, Factually

Maybe it's an unsolvable paradox but it could be a better managed one. If we could face up to this conflicting reality we'd be potentially better equipped at avoiding situations where we are likely to give in to the temptation of lust.

Infidelity research shows people make lots of small decisions and then suddenly they're in a position where their willpower isn't up to much, for example: being drunk and alone with a co-worker you have a certain spark with at Friday night drinks. "When you're thinking about crossing the line it's easy to ignore the possibly really massive negative consequences. And if people had that information they'd find it easier to avoid that situation.

"We have really unrealistic expectations and because we don't talk about infidelity because everyone is so judgemental about it, there's no honest conversation about it.

"I wonder what the rates would be if we understood more about why [infidelity] happens, how it happens, what you could do to stop it happening and the impact it has on relationships."

If we could admit, without judgment, that this is a situation where we're more likely to give in to short-term reward, we could avoid the thrill of that temptation in the first place, Mucha says.

Because it is that thrill that we chase. "The parts of the brain that are activated when you're tempted by someone is the same as when you take cocaine and amphetamines."

Polyamory doesn't seem to reduce levels of cheating - partners were still liable to break the rules of their open relationship despite having more scope to sleep with other people. It's the transgression of rule-breaking itself that lures us and that's a difficult problem to solve.

Also tricky to grapple with is the idea of forgiveness. Because of the hurt cheating causes, it's a hard thing to forgive and an easy thing to judge others for doing. Blanket statements won't cover the many variables when it comes to infidelity, but Mucha tends toward thinking forgiving infidelity more often than not leads to better outcomes for all parties.

"Lots of research shows people in mostly happy relationships who break up are less happy seven years later."

I'm sceptical - surely cheating is one of the most unloving things you can do, and is it possible that in some cases it might even be incompatible with claims of love altogether?

"I'm not saying, 'Well look, infidelity happens loads, we should just get over it and forgive everyone,'" Mucha says. "Because that's not right either. But I feel like some of the nuance is missed in talking about it. If you've got rates of infidelity as high as they are, then do none of those people love their partners?"

Mucha thinks it's better to take a "zoomed out" look at the situation. "Is this person, other than the infidelity, are they generally a nice person?" If they are and the relationship is an otherwise happy one then it's worth trying to forgive and move on, she says.

The type of love that sustains very long-term relationships, companionate love, is based on shared values, honesty, realistic expectations and having the others' best interests at heart. While an affair seems the very definition of not having your partner's interests at heart, Mucha strongly believes that if a partner takes proper responsibility for their actions following infidelity and the couple shares a companionate love, infidelity is more than possible to forgive and move on from (taking responsibility is a key factor, and if the person who strayed refuses to then that changes things considerably).

Relationships aren't found, they're built, through an "extraordinary amount of work and commitment", Mucha has learned. "One thing philosophers say, and I agree with, is that how much you believe in commitment impacts how hard you work at relationships." Love isn't just feeling, it's also a decision we make every day to keep working at what we have with the person closest to us; choosing them again and again every day, even when it might seem impossible.